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Mining web graphs for recommendations.bak

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IEEE TRANSACTIONS ON KNOWLEDGE AND DATA ENGINEERING, VOL. 24, NO. 6, JUNE 2012 1051 Mining Web Graphs for Recommendations Hao Ma, Irwin King, Senior Member, IEEE, and Michael Rung-Tsong Lyu, Fellow, IEEE Abstract—As the exponential explosion of various contents generated on the Web, Recommendation techniques have become increasingly indispensable. Innumerable different kinds of recommendations are made on the Web every day, including movies, music, images, books recommendations, query suggestions, tags recommendations, etc. No matter what types of data sources are used for the recommendations, essentially these data sources can be modeled in the form of various types of graphs. In this paper, aiming at providing a general framework on mining Web graphs for recommendations, 1) we first propose a novel diffusion method which propagates similarities between different nodes and generates recommendations; 2) then we illustrate how to generalize different recommendation problems into our graph diffusion framework. The proposed framework can be utilized in many recommendation tasks on the World Wide Web, including query suggestions, tag recommendations, expert finding, image recommendations, image annotations, etc. The experimental analysis on large data sets shows the promising future of our work. Index Terms—Recommendation, diffusion, query suggestion, image recommendation. Ç1 INTRODUCTIONW ITH the diverse and explosive growth of Web Fortunately, on the Web, no matter what types of data information, how to organize and utilize the informa- sources are used for recommendations, in most cases, thesetion effectively and efficiently has become more and more data sources can be modeled in the form of various types ofcritical. This is especially important for Web 2.0 related graphs. If we can design a general graph recommendationapplications since user-generated information is more algorithm, we can solve many recommendation problemsfreestyle and less structured, which increases the difficulties on the Web. However, when designing such a frameworkin mining useful information from these data sources. In for recommendations on the Web, we still face severalorder to satisfy the information needs of Web users and challenges that need to be addressed.improve the user experience in many Web applications, The first challenge is that it is not easy to recommend http://ieeexploreprojects.blogspot.comRecommender Systems, have been well studied in academia latent semantically relevant results to users. Take Queryand widely deployed in industry. Suggestion as an example, there are several outstanding Typically, recommender systems are based on Collabora- issues that can potentially degrade the quality of thetive Filtering [14], [22], [25], [41], [46], [49], which is a recommendations, which merit investigation. The first onetechnique that automatically predicts the interest of an is the ambiguity which commonly exists in the naturalactive user by collecting rating information from other language. Queries containing ambiguous terms may con-similar users or items. The underlying assumption of fuse the algorithms which do not satisfy the informationcollaborative filtering is that the active user will prefer needs of users. Another consideration, as reported in [26]those items which other similar users prefer [38]. Based on and [53], is that users tend to submit short queriesthis simple but effective intuition, collaborative filtering has consisting of only one or two terms under most circum-been widely employed in some large, well-known com- stances, and short queries are more likely to be ambiguous.mercial systems, including product recommendation at Through the analysis of a commercial search engine’s queryAmazon,1 movie recommendation at Netflix,2 etc. Typical logs recorded over three months in 2006, we observe thatcollaborative filtering algorithms require a user-item rating 19.4 percent of Web queries are single term queries, andmatrix which contains user-specific rating preferences to further 30.5 percent of Web queries contain only two terms.infer users’ characteristics. However, in most of the cases, Third, in most cases, the reason why users perform a searchrating data are always unavailable since information on the is because they have little or even no knowledge about theWeb is less structured and more diverse. topic they are searching for. In order to find satisfactory answers, users have to rephrase their queries constantly. 1. http://www.amazon.com. 2. http://www.netflix.com. The second challenge is how to take into account the personalization feature. Personalization is desirable for many scenarios where different users have different. H. Ma is with The Chinese University of Hong Kong, Room 101, HSH information needs. As an example, Amazon.com has been Engineering Building, Shatin, N.T., Hong Kong. the early adopter of personalization technology to recom- E-mail: hma@cse.cuhk.edu.hk.. I. King and M.R. Lyu are with the Department of Computer Science and mend products to shoppers on its site, based upon their Engineering, The Chinese University of Hong Kong, Shatin, N.T., Hong previous purchases. Amazon makes an extensive use of Kong. E-mail: {king, lyu}@cse.cuhk.edu.hk. collaborative filtering in its personalization technology. TheManuscript received 15 Apr. 2009; revised 6 Apr. 2010; accepted 6 Aug. 2010; adoption of personalization will not only filter outpublished online 23 Dec. 2010. irrelevant information to a person, but also provide moreFor information on obtaining reprints of this article, please send e-mail to:tkde@computer.org, and reference IEEECS Log Number TKDE-2009-04-0350. specific information that is increasingly relevant to aDigital Object Identifier no. 10.1109/TKDE.2011.18. person’s interests. 1041-4347/12/$31.00 ß 2012 IEEE Published by the IEEE Computer Society

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1052 IEEE TRANSACTIONS ON KNOWLEDGE AND DATA ENGINEERING, VOL. 24, NO. 6, JUNE 2012 The last challenge is that it is time consuming and filtering generally can achieve higher performance than theinefficient to design different recommendation algorithms other popular algorithm VSS, since it considers thefor different recommendation tasks. Actually, most of these differences of user rating style.recommendation problems have some common features, In the model-based approaches, training data sets arewhere a general framework is needed to unify the used to train a predefined model. Examples of model-basedrecommendation tasks on the Web. Moreover, most of approaches include the clustering model [33], the aspectexisting methods are complicated and require to tune a models [23], [24], [52] and the latent factor model [9]. Kohrslarge number of parameters. and Merialdo [33] presented an algorithm for collaborative In this paper, aiming at solving the problems analyzed filtering based on hierarchical clustering, which tried toabove, we propose a general framework for the recommen- balance robustness and accuracy of predictions, especiallydations on the Web. This framework is built upon the heat when few data were available. Hofmann [23] proposed andiffusion on both undirected graphs and directed graphs, algorithm based on a generalization of probabilistic latentand has several advantages. semantic analysis to continuous-valued response variables. Recently, several matrix factorization methods [39], [40], 1. It is a general method, which can be utilized to many [46], [48], [49], [54] have been proposed for collaborative recommendation tasks on the Web. filtering. These methods all focus on fitting the user-item 2. It can provide latent semantically relevant results to rating matrix using low-rank approximations, and use it to the original information need. make further predictions. The premise behind a low- 3. This model provides a natural treatment for perso- dimensional factor model is that there is only a small nalized recommendations. number of factors influencing preferences, and that a user’s 4. The designed recommendation algorithm is scalable preference vector is determined by how each factor applies to very large data sets. to that user.The empirical analysis on several large scale data sets (AOL Although collaborative filtering methods have beenclickthrough data and Flickr image tags data) shows that extensively studied recently, most of these methods requireour proposed framework is effective and efficient for the user-item rating matrix. However, on the Web, in mostgenerating high-quality recommendations. of the cases, rating data are always unavailable since The rest of the paper is organized as follows. We review information on the Web is less structured and more diverse.related work in Section 2. Section 3 presents the diffusion Hence, collaborative filtering algorithms cannot be directlymodels on both undirected graphs and directed graphs. In applied to most of the recommendation tasks on the Web,Section 4, we demonstrate the empirical analysis of our like query suggestion and image recommendation. http://ieeexploreprojects.blogspot.commodels and recommendation algorithms on several diver-sified data sources. Finally, conclusion is given in Section 5. 2.2 Query Suggestion In order to recommend relevant queries to Web users, a valuable technique, query suggestion, has been employed2 RELATED WORK by some prominent commercial search engines, such asRecommendation on the Web is a general term representing Yahoo!,3 Live Search,4 Ask,5 and Google.6 However, due toa specific type of information filtering technique that commercial reasons, a few public papers have been releasedattempts to present information items (queries, movies, to reveal the methods they adopt.images, books, Web pages, etc.) that are likely of interest to The goal of query suggestion is similar to that of querythe users. In this section, we review several work related to expansion [11], [13], [56], [61], query substitution [31], andrecommendation, including collaborative filtering, query query refinement [35], [57], which all focus on under-suggestion techniques, image recommendation methods, standing users’ search intentions and improving the queriesand clickthrough data analysis. submitted by users. Query suggestion is closely related to query expansion or query substitution, which extends the2.1 Collaborative Filtering original query with new search terms to narrow down theTwo types of collaborative filtering approaches are widely scope of the search. But different from query expansion,studied: neighborhood-based and model-based. query suggestion aims to suggest full queries that have been The neighborhood-based approaches are the most formulated by previous users so that query integrity andpopular prediction methods and are widely adopted in coherence are preserved in the suggested queries [18].commercial collaborative filtering systems [37], [47]. The Query refinement is another closely related notion, since themost analyzed examples of neighborhood-based collabora- objective of query refinement is interactively recommend-tive filtering include user-based approaches [7], [21] and ing new queries related to a particular query.item-based approaches [15], [37], [50]. User-based ap- In [61], local (i.e., query-dependent documents) andproaches predict the ratings of active users based on the global (i.e., the whole corpus) documents are employed inratings of their similar users, and item-based approaches query expansion by applying the measure of global analysispredict the ratings of active users based on the computed to the selection of query terms in local feedback. Althoughinformation of items similar to those chosen by the active experimental results show that this method is generallyuser. User-based and item-based approaches often use thePearson Correlation Coefficient algorithm (PCC) [47] and 3. http://www.yahoo.com. 4. http://www.live.com.the Vector Space Similarity algorithm (VSS) [7] as the 5. http://www.ask.com.similarity computation methods. PCC-based collaborative 6. http://www.google.com.

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MA ET AL.: MINING WEB GRAPHS FOR RECOMMENDATIONS 1053more effective than global analysis, it performs worse than well studied in the query clustering problem [5], [60]. Querythe query expansion method proposed in [13] based on user clustering is a process used to discover frequently askedinteractions recorded in user logs. In another approach questions or most popular topics on a search engine. Thisreported in [35], anchor texts are employed for the purpose process is crucial for search engines based on question-of query refinement. This work is based on the observation answering [60]. Recently, clickthrough data has beenthat Web queries and anchor texts are highly similar. These analyzed and applied to several interesting research topics,methods employ different kinds of data sources (docu- such as Web query hierarchy building [51] and extraction ofments, anchor texts, query logs, etc.) for suggesting queries. class attributes [44]. In [51], the proposed method consists ofSince most of these methods are only designed for query two stages: generating candidate queries and determiningsuggestions, the extensibility of these methods are very “generalization/specialization” relations between theselimited. In [4] and [16], two query recommendation queries in a hierarchy. A typical relationship can bemethods based on clickthrough data are proposed. The learning from clickthrough data is that “bmw” is a childmain disadvantage of these two algorithms is that they of “car.” The method proposed in [44] can extract attributes such as “capital city” and “President” for the classignore the rich information embedded in the query-click 7 “Country,” or “cost,” “manufacturer” and “side effects”bipartite graph, and consider only queries that appear in for the class “Drug.” The method initially relies on a smallthe query logs, potentially losing the opportunity to set of linguistically motivated extraction patterns applied torecommend highly semantically related queries to users. each entry from the query logs, then employs a series ofCao et al. [10] developed a context-aware query suggestion Web-based precision-enhancement filters to refine and rankmethod by mining clickthrough and session data. This work the candidate attributes.first extracts some concepts from the clickthrough data bybuilding clusters. Then, these concepts as well as the query 2.4 Image Recommendationsessions are employed to build a concept sequence suffix Besides query suggestion, another interesting recommenda-tree for query suggestion. Recently, Mei et al. proposed a tion application on the Web is image recommendation.general query suggestion method using hitting time on the Image recommendation systems, like Photoree,8 focus onquery-click bipartite graph in [42]. This method can recommending interesting images to Web users based ongenerate semantically relevant queries to users’ information users’ preference. Normally, these systems first ask users toneeds. The main advantage of this work is that it can rate some images as they like or dislike, and thensuggest some long tail queries (infrequent queries) to users. recommend images to the users based on the tastes of theHowever, this is also the disadvantage of this approach users. In the academia, a few tasks are proposed to solve thesince sometimes it may accidentally rank the infrequent image recommendation problems since this is a relatively http://ieeexploreprojects.blogspot.comqueries highly in the results while potentially downgrades new field and analyzing the image contents is a challengethe ranks of the most related queries. job. Recently, in [63], by employing the Flickr data set, Yang Actually, as reported in [42], several different ranking et al. proposed a context-based image search and recom-methods using random walks can also be employed into the mendation method to improve the image search quality andquery suggestion tasks on a query-URL bipartite graph, recommend related images and tags. However, since it is aincluding PageRank [8], HITS [32], etc. PageRank is context-based method, the computational complexity isbasically computing the stationary distribution of a very high and it cannot scale to large data sets. While in oursmoothed Markov chain. Personalized PageRank gener- framework proposed in this paper, by diffusing on thealizes PageRank by smoothing the Markov chain with a image-tag bipartite graph with one or more images, we canquery-specific jumping probability vector instead of a accurately and efficiently suggest semantically relevantuniform vector, thus, is often used for query-dependent nonpersonalized or personalized images to the users.ranking [19], [20], [28]. HITS is an alternative query- In general, comparing with previous work, our work isdependent ranking algorithm which computes hub and a general framework which can be effectively, efficiently,authority scores in an iterative way. In [12], the query and naturally applied to most of the recommendation taskssuggestion and the document retrieval problem are inter- on the Web.preted using the Markov random walks, in which thequeries or documents with the largest probabilities after t-step random walks are recommended to the users. 3 DIFFUSION ON GRAPHS2.3 Clickthrough Data Analysis In this section, we first introduce a novel graph diffusionIn the field of clickthrough data analysis, the most common model based on heat diffusion. This model can be appliedusage is for optimizing Web search results or rankings [1], to both undirected graphs and directed graphs. We then[29], [30], [55], [59]. In [59], Web search logs are utilized to present how to infer the parameter based on the grapheffectively organize the clusters of search results by structure. Last, we analyze the computational complexity of1) learning “interesting aspects” of a topic and 2) generating our model.more meaningful cluster labels. In [30], a ranking function islearned from the implicit feedback extracted from search 3.1 Heat Diffusionengine clickthrough data to provide personalized search Heat diffusion is a physical phenomenon. In a medium,results for users. Besides ranking, clickthrough data is also heat always flows from a position with high temperature to a position with low temperature. Recently, heat 7. The query-click graph is a bipartite graph with queries on one side andclicked documents on the other. 8. http://www.photoree.com.

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1054 IEEE TRANSACTIONS ON KNOWLEDGE AND DATA ENGINEERING, VOL. 24, NO. 6, JUNE 2012diffusion-based approaches have been successfully ap- andplied in various domains such as classification and dimensionality reduction problems [6], [34], [36]. Lafferty dðvi Þ; i ¼ j; Dij ¼ ð4Þand Lebanon [36] approximated the heat kernel for a 0; otherwise;multinomial family in a closed form, from which great where dðvi Þ is the degree of node vi . From the definition, theimprovements were obtained over the use of Gaussian or matrix D is a diagonal matrix.linear kernels. In [34], Kondor and Lafferty proposed theuse of a discrete diffusion kernel for categorical data, and In order to generate a more generalized representation, weshowed that the simple diffusion kernel on the hypercube normalize all the entries in matrices H and D by the degree ofcan result in good performance for such data. Belkin and each node. The matrices H and D can be modified toNiyogi employed a heat kernel to construct the weight of 8 < 1=dðvi Þ; ðvi ; vj Þ 2 E;a neighborhood graph, and apply it to a nonlinear Hij ¼ 0; i ¼ j; ð5Þdimensionality reduction algorithm in [6]. In [62], Yang : 0; otherwise;et al. proposed a ranking algorithm known as theDiffusionRank using heat diffusion process; simulations andshowed that it is very robust to Web spamming. In this paper, we use heat diffusion to model the 1; i ¼ j; Dij ¼ ð6Þsimilarity information propagation on Web graphs. In 0; otherwise:Physics, the heat diffusion is always performed on a In the limit Át ! 0, this becomesgeometric manifold with initial conditions. However, it isvery difficult to represent the Web as a regular geometry dwith a known dimension. This motivates us to investigate f ðtÞ ¼ tðH À DÞf ðtÞ: ð7Þ dtthe heat flow on a graph. The graph is considered as an Solving this differential equation, we haveapproximation to the underlying manifold, and so the heatflow on the graph is considered as an approximation to the f ð1Þ ¼ eðHÀDÞ f ð0Þ; ð8Þheat flow on the manifold. where dðvÞ denotes the degree of the node v, and eðHÀDÞ3.2 Diffusion on Undirected Graphs could be extended asConsider an undirected graph G ¼ ðV ; EÞ, where V is thevertex set, and V ¼ fv1 ; v2 ; . . . ; vn g. E ¼ fðvi ; vj Þ j there is an 2 eðHÀDÞ ¼ I þ ðH À DÞ þ ðH À DÞ2 http://ieeexploreprojects.blogspot.comedge between vi to vj g is the set of all edges. The edge ðvi ; vj Þ 2! ð9Þis considered as a pipe that connects nodes vi and vj . The 3 3 þ ðH À DÞ þ Á Á Á :value fi ðtÞ describes the heat at node vi at time t, beginning 3!from an initial distribution of heat given by fi ð0Þ at time The matrix eðHÀDÞ is called the diffusion kernel in the sensezero. f ðtÞ denotes the vector consisting of fi ðtÞ. that the heat diffusion process continues infinitely many We construct our model as follows: suppose, at time t, times from the initial heat diffusion.each node i receives an amount Mði; j; t; ÁtÞ of heat from its In order to interpret (8) and the heat diffusion processneighbor j during a time period Át. The heat Mði; j; t; ÁtÞ more intuitively, we construct a small undirected graphshould be proportional to the time period Át and the heat with only five nodes as showed in Fig. 1a.difference fj ðtÞ À fi ðtÞ. Moreover, the heat flows from node Initially, at time zero, suppose node 1 is given 3 units ofj to node i through the pipe that connects nodes i and j. heat, and node 2 is given 2 units of heat; then the vector f ð0ÞBased on this consideration, we assume that Mði; j; t; ÁtÞ ¼ equals ½3; 2; 0; 0; 0T . The entries in matrix H À D areðfj ðtÞ À fi ðtÞÞÁt, where is the thermal conductivity—the 0 1heat diffusion coefficient. As a result, the heat difference at À1 1 1 1 1node i between time t þ Át and time t will be equal to the B 1 À1 0 0 0 C B 4 Csum of the heat that it receives from all its neighbors. This is HÀD¼B 4 B 1 0 À1 0 0 C: Cformulated as @ 1 0 0 À1 0 A 4 1 0 0 0 À1 fi ðt þ ÁtÞ À fi ðtÞ X 4 ¼ ðfj ðtÞ À fi ðtÞÞ; ð1Þ Without loss of generality, we set the thermal conduc- Át j:ðvj ;vi Þ2E tivity ¼ 1, and vary time t from 0 to 1 with a step of 0.05.where E is the set of edges. To find a closed form solution to The curve for the amount of heat at each node with time is(1), we express it in a matrix form shown in Fig. 1b. We can see that, as time passes, the heat sources nodes 1 and 2 will diffuse their heat to nodes 3, 4, f ðt þ ÁtÞ À f ðtÞ and 5. The heat of nodes 3, 4, and 5 will increase ¼ ðH À DÞf ðtÞ; ð2Þ respectively, and the trends of their heat curves are the Át same since these three nodes are symmetric in this graph.where Another example is shown in Fig. 1c. Initially, at time 8 zero, suppose node 1 is given 4 units of heat,then the vector 1; ðvi ; vj Þ 2 E or ðvj ; vi Þ 2 E; T Hij ¼ 0; i ¼ j; ð3Þ f ð0Þ equals ½4; 0; 0; 0 . The related heat curve is shown in : Fig. 1b. We can see that the node 2, the closest node to the 0; otherwise; heat source, gains more heat than other nodes. This also

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MA ET AL.: MINING WEB GRAPHS FOR RECOMMENDATIONS 1055 2. The heat DHði; t; ÁtÞ should be proportional to the heat at node vi . 3. Each node has the same ability to diffuse heat. 4. The heat DHði; t; ÁtÞ should be proportional to the weight assigned between node vi and its subsequent nodes. As a result, node vi will diffuse wij fi ðtÞÁt= P k:ði;kÞ2E wik amount of heat to each of its subsequent nodes vj , and each vj should receive wij fi ðtÞÁt= P k:ði;kÞ2E wik amount of heat from node vi . P Therefore, j ¼ wji = k:ðj;kÞ2E wjk . In the case that the outdegree of node vi equals zero, we assume that this node will not diffuse heat to others. To sum up, the heat difference at node vi between time t þ Át and t will be equal to the sum of the heat that it receives, deducted by what it diffuses. This is formulated as fi ðt þ ÁtÞ À fi ðtÞ Át 0 1 X ð10Þ wji ¼ @Ài fi ðtÞ þ P fj ðtÞA; j:ðvj ;vi Þ2E k:ðj;kÞ2E wjkFig. 1. Two simple heat diffusion examples on an undirected graph.(a) Example 1. (b) Curve of heat change with time. (c) Example 2. where i is a flag to identify whether node vi has any(d) Curve of heat change with time. outlinks. Solving it, we obtainindicates that if a node has more paths connected to the heat f ð1Þ ¼ eðHÀDÞ f ð0Þ; ð11Þsource, it will potentially obtain more heat. This is a perfectproperty for recommending relevant nodes on a graph. where 8 ,3.3 Diffusion on Directed Graphs X wji wjk ; ðvj ; vi Þ 2 E;The above heat diffusion model is designed for undirected http://ieeexploreprojects.blogspot.com k:ðj;kÞ2E Hij ¼ 0; ð12Þgraphs, but in many situations, the Web graphs are i ¼ j; :directed, especially in online recommender systems or 0; otherwise;knowledge sharing sites. Every user in knowledge sharingsites typically has a trust list. The users in the trust list can andinfluence this user deeply. These relationships are directed i ; i ¼ j;since user a is in the trust list of user b, but user b might not Dij ¼ ð13Þ 0; otherwise:be in the trust list of user a. At the same time, the extent oftrust relations is different since user ui may trust user uj 3.4 Random Jumpwith trust score 1 while trust user uk only with trust score The heat can only propagate through the links that connect0.2. Hence, there are different weights associated with the nodes in a given graph, but in fact, there are randomrelations. Based on this consideration, we modify the heat relations among different nodes even if these nodes are notdiffusion model for the directed graphs as follows. connected. For an example, in the clickthrough data, people Consider a directed graph G ¼ fV ; E; W g, where V is the of different cultures, genders, ages, and environments, mayvertex set, and V ¼ fv1 ; v2 ; . . . ; vn g. W ¼ fwij j where wij is implicitly link queries together, but we do not know thesethe probability that edge ðvi ; vj Þ existsg or the weight that is latent relations. Another good example is the trust relationsassociated with this edge. E ¼ fðvi ; vj Þ j there is an edge in a social network. On online social network sites, usersfrom vi to vj and wij 0g is the set of all edges. always explicitly state the trust relations to other users. On a directed graph GðV ; EÞ, in the pipe ðvi ; vj Þ, heat Actually, there are some other implicit hidden trustflows only from vi to vj . Suppose at time t, each node vi relations among these users that cannot be observed.receives RH ¼ RHði; j; t; ÁtÞ amount of heat from vj during Hence, to capture these relations, we propose to add aa period of Át. We make three assumptions: 1) RH should uniform random relation among different nodes. Morebe proportional to the time period Át; 2) RH should be specifically, let denote the probability that such phenom-proportional to the heat at node vj ; and 3) RH is zero if ena happen, and ð1 À Þ is the probability of taking a “random jump.” Without any prior knowledge, we setthere is no link from vj to vi . As a result, vi will receiveP 1 g ¼ n 1, where g is a uniform stochastic distribution vector, j:ðvj ;vi Þ2E j fj ðtÞÁt amount of heat from all its neighbors 1 is the vector of all ones, and n is the number of nodes.that point to it. Based on the above consideration, we modify our model to At the same time, node vi diffuses DHði; t; ÁtÞ amount ofheat to its subsequent nodes. We assume that f ð1Þ ¼ eR f ð0Þ; R ¼ ðH À DÞ þ ð1 À Þg1T : ð14Þ 1. The heat DHði; t; ÁtÞ should be proportional to the Following the setting of in PageRank [17], [43], we set time period Át. ¼ 0:85 in all of our experiments conducted in Section 4.

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1056 IEEE TRANSACTIONS ON KNOWLEDGE AND DATA ENGINEERING, VOL. 24, NO. 6, JUNE 2012 TABLE 1 Samples of Search Engine Clickthrough Data3.5 Complexity Analysis users’ information need. In this section, we demonstrateWhen the graph is very large, a direct computation of eR is how our method can benefit the query suggestion, and howvery time consuming. We adopt its discrete approximation to mine latent semantically similar queries based on theto compute the heat diffusion equation users’ information need. P f ð1Þ ¼ I þ R f ð0Þ; ð15Þ 4.1.1 Data Collection P We construct our query suggestion graph based on thewhere P is a positive integer. In order to reduce the clickthrough data of the AOL search engine [45]. In total, thiscomputational complexity, we introduce two techniques: data set spans 3 months from 01 March, 2006 to 31 May, 2006.1) since f ð0Þ is a vector, we iteratively calculate ðI þ There are a total of 19,442,629 lines of clickthrough informa- P P RÞ f ð0Þ by applying the operator ðI þ P RÞ to f ð0Þ; 2) for tion, 4,802,520 unique queries, and 1,606,326 unique URLs.matrix R, we employ a data structure which only stores Clickthrough data record the activities of Web users,the information of nonzero entries, since it is a very sparse which reflect their interests and the latent semanticmatrix. Thus, supposing a graph is connected by M edges relationships between users and queries as well as queries(relationships between nodes), the complexity of executing and clicked Web documents. As shown in Table 1, eachthe heat diffusion process is OðP MÞ, which represents the line of clickthrough data contains the following informa-number of iterations P multiplied by the number of edges tion: a user ID (u), a query (q) issued by the user, a URL (l)M in a graph. In most cases, P ¼ 10 is enough for on which the user clicked, the rank (r) of that URL, and theapproximating the heat diffusion equation. The complexity time (t) at which the query was submitted for search. Thus, http://ieeexploreprojects.blogspot.comOðP MÞ shows that our heat diffusion algorithm enjoys the clickthrough data can be represented by a set ofvery good performance in scalability since it is linear with quintuples hu; q; l; r; ti. From a statistical point of view, therespect to the number of edges in the graph. query word set corresponding to a number of Web pages However, since the size of Web information is very large, contains human knowledge on how the pages are relatedthe graph built upon the Web information can become to their issued queries [55]. Thus, in this paper, we utilizeextremely large. Then, the complexity OðP MÞ is also too the relationships of queries and Web pages for thehigh, and the algorithm becomes time consuming and construction of the bipartite graph containing two typesinefficient to get a solution. To overcome this difficulty, we of vertices hq; li. The information regarding user ID, rankfirst extract a subgraph starting from the heat sources. and calendar time is ignored.Given the heat sources, the subgraph is constructed by This data set is the raw data recorded by the searchusing depth-first search in the original graph. The search engine, and contains a lot of noise which will potentiallystops when the number of nodes is larger than a predefined affect the effectiveness of our query suggestion algorithm.number. Then, the diffusion processes will be performed onthis subgraph efficiently and effectively. Generally, it will Hence, we conduct a similar method employed in [59] tonot decrease the qualities of the heat diffusion processes clean up the raw data. We filter the data by only keepingsince the nodes too far away from the heat sources are those frequent, well formatted, English queries (queriesnormally not related to the sources. which only contain characters “a,” “b,” . . . , “z,” and space). After cleaning and removing duplicates, we get totally 2,019,265 unique queries and 915,771 unique URLs in our4 EMPIRICAL ANALYSIS data collection. After the construction of the query-URLIn Section 3, we introduced our graph diffusion models for bipartite graph using this data collection procedure, werecommendations. In this section, 1) we show how to observe that a total of 7,633,400 edges exist in the query-convert different Web data sources into correct graphs in URL bipartite graph, which indicates that, on average, eachour models; and 2) we conduct several experiments on query has 3.78 distinct clicks, and each URL is clicked byquery suggestions, and image recommendations.9 8.34 distinct queries.4.1 Query Suggestion 4.1.2 Graph ConstructionQuery Suggestion is a technique widely employed by For the query-URL bipartite graph, consider an undirectedcommercial search engines to provide related queries to bipartite graph Bql ¼ ðVql ; Eql Þ, where Vql ¼ Q [ L, Q ¼ fq1 ; q2 ; . . . ; qn g, and L ¼ fl1 ; l2 ; . . . ; lp g. Eql ¼ fðqi ; lj Þj there is an 9. All the experiments are computed by a workstation consisting of twoIntel Xeon CPUs (2.5 GHz, Quad-Core) and 8 Giga memories. The operating edge from qi to lj g is the set of all edges. The edge ðqj ; lk Þ existssystem is Windows Server 2003, 32-bit. if and only if a user ui clicked a URL lk after issuing a query qj .

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MA ET AL.: MINING WEB GRAPHS FOR RECOMMENDATIONS 1057 to 1, and the size of the subgraph is set to 5,000. From the suggestions, we can see that the query suggestions generated by our method are generally as good as those from commercial search engines. For some queries, our suggestions are even better. In order to compare our method with other approaches, we create a set of 200 queries as the testing queries, covering a wide range of topics, such as Computers, Arts, Business, and others. Some of the results generated by our DRec algorithm are shown in Table 3. From the results, we observe that our recommendation algorithm not only suggests queries which are literally similar to the test queries, but also provides latent semantically relevant recommendations. For instance, if the test query is a technique, such as “java,” we recommendFig. 2. Graph construction for query suggestion. (a) Query-URL bipartite “virtual machine” and “sun microsystems.” The lattergraph. (b) Converted query-URL bipartite graph. suggestion is the company who owns the Java Platform, and the former suggestion is a key feature of the JavaSee Fig. 2a for an example. The values on the edges in Fig. 2a programming language. They both have high latent seman-specify how many times a query is clicked on a URL. tic relations to the query “java.” If the test query is a human We cannot simply employ the bipartite graph extracted name, such as “michael jordan,” one of the most successfulfrom the clickthrough data into the diffusion processes NBA basketball player, the latent semantic suggestions aresince this bipartite graph is an undirected graph, and “nba,” “nike,” “jordan xi” (a model of Air Jordan shoes), andcannot accurately interpret the relationships between “air jordans,” All of the results show that our latent semanticqueries and URLs. Hence, we convert this bipartite graph query suggestion algorithm has a promising future.into Fig. 2b. In this converted graph, every undirected edge Since the data set we use is different from the data setsin the original bipartite graph is converted into two directed that these commercial search engines employ, it is difficultedges. The weight on a directed query-URL edge is to quantitatively evaluate our results with those from thenormalized by the number of times that the query is issued, commercial search engines objectively. Hence, we comparewhile the weight on a directed URL-query edge is normal- our DRec method with the baseline approaches using http://ieeexploreprojects.blogspot.comized by the number of times that the URL is clicked. SimRank [27], Forward Random Walk (FRW) [12], and Backward Random Walk (BRW) [12].4.1.3 Query Suggestion Algorithm In the method of SimRank, we use the query-URLAfter the conversion of the graph, we can easily design the bipartite graph to calculate the similarities between queries.query suggestion algorithm in Algorithm 1. Then, based on the similarities, we recommend the top-5 similar queries to users. SimRank is based on the intuitionAlgorithm 1. Query Suggestion Algorithm that two queries are very similar if they link to a lot of 1: A converted bipartite graph G ¼ ðV þ [ V Ã ; EÞ consists similar URLs. On the other hand, two URLs are very similar of query set V þ and URL set V Ã . The two directed if they are clicked as a result of several similar queries. edges are weighted using the method introduced Based on this intuition, in SimRank, we first calculate the in previous section. similarities between URLs, then we compute the similarities 2: Given a query q in V þ , a subgraph is constructed by for queries based on the similarities of URLs. We iteratively using depth-first search in G. The search stops when update the similarities until they converge. the number of queries is larger than a predefined In FRW and BRW methods, we perform forward random number. walk and backward random walk starting from the query 3: As analyzed above, set ¼ 1, and without loss of node on the query-URL graph. After the random walks, we generality, set the initial heat value of query use the top-ranked queries as the suggestions. q fq ð0Þ ¼ 1 (the choice of initial heat value will not Evaluating the quality of semantic relations is difficult, in affect the suggestion results). Start the diffusion particular for the contents generated by users, as there are no process using linguistic resources available. In this paper, we conduct both f ð1Þ ¼ eR f ð0Þ. a manual evaluation by a panel of three human experts, and an automatic evaluation based on the ODP10 database. 4: Output the Top-K queries with the largest values in In the evaluation by human experts, the three experts are vector f ð1Þ as the suggestions. three PhD students without any overlaps with the authors. They do not know which algorithms are tested. They are also not allowed to communicate with each other during the4.1.4 Query Suggestion Results evaluation process. We ask all the experts to rate the queryWe display the suggestion results of our algorithm and suggestion results. We define a 6-point scale (0, 0.2, 0.4, 0.6,those from Google, Yahoo!, Live Search, and AOL in Table 2. 0.8, and 1) to measure the relevance between the testingWe call our algorithm DRec which means Recommenda-tions by Diffusion. In our algorithms, the parameter is set 10. http://www.dmoz.org.

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MA ET AL.: MINING WEB GRAPHS FOR RECOMMENDATIONS 1059 TABLE 3 Examples of DRec Query Suggestion Results (k ¼ 50)queries and the suggested queries, in which 0 means above is 4=5 since they share the path “Computers :“totally irrelevant” while 1 indicates “entirely relevant.” Programming : Languages : Java” and the longest one isThe average values of evaluation results are shown in Fig. 3. made of five directories. We have evaluated the similarityWe observe that, when measuring the results by human between two queries by measuring the similarity betweenexperts, our DRec algorithm increases the accuracy for the most similar categories of the two queries, among theabout 19.81, 13.0, and 7.5 percent comparing with the top five answers provided by ODP.SimRank, BRW, and FRW algorithm, respectively. As shown in Fig. 4, we observe that, when evaluating For the automatic evaluation, we utilize the ODP using ODP database, our proposed DRec algorithm increasesdatabase. ODP, also known as dmoz, is one of the largest, the suggestion accuracy for about 22.45, 11.9, and 7.5 percentmost comprehensive human-edited directories of the Web. comparing with the SimRank, BRW, and FRW algorithm,In our experiment, we adopt the same method used in [3] to respectively. This indicates again that our proposed queryevaluate the quality of the suggested queries. When a user suggestion algorithm is very effective. The major differencetypes a query in ODP, besides site matches, we can also find between our proposed heat diffusion model and random walk model is that heat diffusion model has the definitions ofcategories matches in the form of paths between directories. thermal conductivity and time frame. Heat will diffuse from http://ieeexploreprojects.blogspot.com based on thermal conductivity andMoreover, these categories are ordered by relevance. For one node to another nodeinstance, the query “Java” would provide the hierarchical time frame step by step, which means heat diffusion is acategory “Computers : Programming : Languages : Java,” relatively slow diffusion process. However, in the case ofwhere “:” is used to separate different categories. One of the random walk, the diffusion from one node to its neighbors isresults for “Virtual Machine” would be “Computers : done immediately. Hence, in our model, after the diffusionProgramming : Languages : Java : Implementations.” process, the recommended items will preserve more in-Hence, to measure how related two queries are, we can formation of the original node. That is also why the resultsuse a notion of similarity between the corresponding outperform random walk models.categories (as provided by ODP). In particular, we measurethe similarity between two categories D and D0 as the length 4.1.5 Impact of Parameter of their longest common prefix F ðD; D0 Þ divided by the The parameter plays an important role in our method. Itlength of the longest path between D and D0 . More controls how fast heat will propagation on the graph.precisely, denoting the length of a path with jDj, this Hence, we also conduct experiments on evaluating thesimilarity is defined as SimðD; D0 Þ ¼ jF ðD; D0 Þj=maxfjDj; impact of parameter . The evaluation results are shown injD0 jg. For instance, the similarity between the two queries Fig. 5. We can observe that the best setting is 1. If weFig. 3. Accuracy comparisons measured by experts. Fig. 4. Accuracy comparisons measured by ODP.

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1060 IEEE TRANSACTIONS ON KNOWLEDGE AND DATA ENGINEERING, VOL. 24, NO. 6, JUNE 2012Fig. 5. Impact of (subgraph size is 5,000). Fig. 6. Impact of the size of subgraph ( ¼ 1).choose relatively smaller thermal conductivity, the perfor- photographs, and communicate with other users. As ofmance will drop since some relevant nodes cannot get November 2007, it claims to host more than 2 billion imagesenough heat. On the other hand, if we choose relatively [2]. Hence, Flickr is an ideal source for the investigation oflarger value of , the performance will also decrease. This isimage-related research.because if the heat transfers very fast, some irrelevant nodes With this Flickr data set, we can apply our recommenda-will gain more heat, hence will hurt the performance. tion framework into several application areas, including Image-to-Image Recommendation, Image-to-Tag Sugges-4.1.6 Impact of the Size of Subgraph tion, Tag-to-Image Retrieval, and Tag-to-Tag Suggestion.As mentioned in Section 3.5, due to the reason that Web In this section, we will only show the performance of ourgraphs are normally very huge, we will perform our model on the image-to-image recommendation application.algorithm on a subgraph extracted from the original graph. For the research of image recommendation, via the FlickrHence, it is necessary to evaluate how the size of this API, we have crawled related information of 36,450,736subgraph affects the recommendation accuracy. Fig. 6 images, which spans from January 1 2007 to December 31shows the performance changes with different subgraph 2007. For each image, we record the following information: http://ieeexploreprojects.blogspot.comsizes. We observe that when the size of the graph is very the user ID who submitted the image, the image ID, thesmall, like 500, the performance of our algorithm is not very time when the image was uploaded, and the tags associatedgood since this subgraph must ignore some very relevant with the image. Totally, we find 807,008 unique users,nodes. When the size of subgraph is increasing, the 5,362,213 unique tags and 322,935,565 edges (tag assign-performance also increases. We also notice that the ments) between images and tags.performance on subgraph with size 5,000 is very close tothe performance with size 100,000. This indicates that the 4.2.2 Image Recommendationnodes that are far away from the query node are normally Basically, the graph construction for image recommenda-not relevant with the query node. tion is similar to the one introduced in Section 4.1. The only4.1.7 Efficiency Analysis difference is that here the nodes in bipartite graph are images and tags, respectively. By using the similarAs analyzed in Section 3.5, our algorithm is very efficient, algorithm which is introduced in Algorithm 1, we can alsoand can be applied to large data sets. Our algorithm has provide image recommendations. The recommendationsimilar complexity with FRW and BRW methods. The results are shown in Fig. 7.computation time for the query suggestion task of these In Fig. 7, we perform three image recommendations. Thethree method (subgraph size is 5,000) is normally around first recommendation is based on the image shown in Fig. 7a,0.10 seconds. However, SimRank is not very efficient since which is a picture taken from Grand Canyon, a national parkit has a high computational complexity. It takes more than in the United States. Figs. 7b to 7f are the five recommenda-15 minutes to compute a query suggestion task in our tions to this picture. We can observe that these recommenda-data set. tions are all latent semantically related to the original picture. This shows the effectiveness and the promising future of our4.2 Image Recommendation DRec method. Figs. 7g and 7m are another two examples,Finding effective and efficient methods to search and with Fig. 7h-Fig. 7i and Fig. 7n-Fig. 7r as the recommenda-retrieve images on the Web has been a prevalent line of tions, respectively. The results also show very goodresearch for a long time [58]. The situation is even tougher performance of our approach, especially for Fig. 7g. Thein the research of Image Recommendation. In this section, we China Great Wall in this query photo is very dark and we canpresent how to recommend related images to the given hardly recognize the object in the photo, while our methodimages using Flickr data set. still can recommend very clear and related results to users, all of which are scenes from Great Wall.4.2.1 Data Collection In order to evaluate the quality of our image recommen-We use Flickr, a popular image hosting Web site and online dation results, we randomly selected 200 images andcommunity for users to share personal photographs, tag compare the results generated by our DRec algorithm with

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MA ET AL.: MINING WEB GRAPHS FOR RECOMMENDATIONS 1061Fig. 7. Examples for image recommendations. (a) Seed image 1. (b) Suggestion 1. (c) Suggestion 2. (d) Suggestion 3. (e) Suggestion 4.(f) Suggestion 5. (g) Seed image 2. (h) Suggestion 1. (i) Suggestion 2. (j) Suggestion 3. (k) Suggestion 4. (l) Suggestion 5. (m) Seed image 3.(n) Suggestion 1. (o) Suggestion 2. (p) Suggestion 3. (q) Suggestion 4. (r) Suggestion 5.those generated by SimRank, FRW, and BRW approaches. performed in this section, we only employ one node (eitherAll the results are evaluated by three experts using a 6-point a query or an image) as the heat source. In the Fig. 8, we can observe personalized image recommendations, we can set all thescale (0, 0.2, 0.4, 0.6, 0.8, and 1). Fromhttp://ieeexploreprojects.blogspot.comthat our DRec method consistently performs better than images submitted by a specified user as the heat sources,SimRank, FRW, and BRW. and then start the diffusion process. This ensures that the If we use the tags instead of the images as the diffusion suggested images are of interests of this user.sources, then this problem turns to be the problem of tag In order to evaluate the quality of our personalized image recommendation method, we create 10 groups:recommendations. Since the recommendation processes are Given1, Given2, . . . , and Given10, where Given1 means inthe same, we do not discuss the results in this paper. this group, all the users only submitted 1 images. We then4.2.3 Personalized Image Recommendation randomly select 50 users from the user list for each group, hence totally we have 500 users. For each of these users, wePersonalization is becoming more and more important in start the diffusion processes once with the submittedmany applications since it is the best way to understand images as the heat sources. After generating the results,different information needs from different users. we ask three experts to rate these recommendations. We Actually, our method can be easily extended to the again define a 6-point scale (0, 0.2, 0.4, 0.6, 0.8, and 1) topersonalized image recommendations. In the query sug- measure the relevance between the testing images and thegestions conducted in Section 4.1 and image suggestions suggested images, in which 0 means “totally irrelevant” while 1 indicates “entirely relevant.” The average values of evaluation results for each group are reported in Fig. 9. WeFig. 8. Accuracy comparisons measured by experts in imagerecommendation. Fig. 9. Accuracy of personalized image recommendations.

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1062 IEEE TRANSACTIONS ON KNOWLEDGE AND DATA ENGINEERING, VOL. 24, NO. 6, JUNE 2012 TABLE 4 Search Results Improvementcan observe that our method generally produces high of Web 2.0 applications, social-based applications gain lotsquality results, and as the number of images increases, the of traffics on the Web. Social recommendation, whichrecommendation quality also increases. produces recommendations by incorporating users’ social network information, is becoming to be an indispensable feature for the next generation of Web applications.5 CONCLUSION The social recommendation problem includes twoIn this paper, we present a novel framework for recom- different data sources, which are social network and user-mendations on large scale Web graphs using heat diffusion. item relation matrices. An example is shown in Fig. 10a. WeThis is a general framework which can basically be adapted can see that in the social network graph, there are trustto most of the Web graphs for the recommendation tasks, scores between different users, while in the user-itemsuch as query suggestions, image recommendations, perso- relation matrix, binary relations connect users and items.nalized recommendations, etc. The generated suggestions We can convert these two graphs into a single andare semantically related to the inputs. The experimental consistent one, as shown in Fig. 10b.analysis on several large scale Web data sources shows the With the constructed graph, for each user (heat source),promising future of this approach. we can start the diffusion process and then recommend the Top-N items to this user. In fact, during the diffusion http://ieeexploreprojects.blogspot.com as shown in Fig. 10b, there are two process on the graph6 FUTURE WORK possible ways to diffuse heat from users to items. The first6.1 Search Results Improvement route is within the user-item bipartite graph, whichIn Table 2, we list the heat values of the suggested queries. captures the intuition that similar users will purchase (orThese values not only can be used in query suggestions, but view) similar items. The second route is passing through thealso are very informative in the advertisement when social network graph, which reflects the social interactionscustomers bid for query terms. Actually, since the diffu- and influences between users. Hence, our DRec diffusionsions are between all the nodes in the graph (including the method naturally fuses these two data sources together fornodes representing queries and the nodes representing social recommendations. We plan to conduct this socialURLs), all the URLs also have heat values. Hence, it is easy recommendation research in the future.to infer that, for a given query, after the diffusion process,the heat values of URLs represent the relatedness to the ACKNOWLEDGMENTSoriginal query, which can also be employed as the ranking The work described in this paper was fully supported byof these URLs. This ranking actually is the wisdom of the two grants from the Research Grants Council of the Hongcrowd since it is based on the query-URL click data, which Kong Special Administrative Region, China (Projects Nos.reflects the intelligent judgements of the Web users. CUHK 412808 and CUHK 415409) and a grant from the The Top-5 Web sites given the queries “sony,” “camera,”“microsoft,” and “chocolate” are shown in Table 4. Forexample, the ranking order for “sony” is different from allof the results retrieved by those four commercial searchengines (which we do not list here due to the spacelimitation). If this order is incorporated into the originalresults, the search results can be greatly improved sincethey are the representations of the implicit votes of all thesearch users. In the future, we plan to compare this rankingmethod with other previous Web search results rankingapproaches, like [59], [64].6.2 Social RecommendationSince our model is quite general, we can apply it to morecomplicated graphs and applications, such as Social Fig. 10. Example for social recommendation. (a) Social network andRecommendation problem. Recently, as the explosive growth user-item relations. (b) Converted graph.

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1064 IEEE TRANSACTIONS ON KNOWLEDGE AND DATA ENGINEERING, VOL. 24, NO. 6, JUNE 2012[47] P. Resnick, N. Iacovou, M. Suchak, P. Bergstrom, and J. Riedl, Irwin King received the BSc degree in en- “Grouplens: An Open Architecture for Collaborative Filtering of gineering and applied science from the Califor- Netnews,” CSCW ’94: Proc. ACM Conf. Computer Supported nia Institute of Technology, Pasadena and the Cooperative Work, 1994. MSc and PhD degrees in computer science from[48] R. Salakhutdinov and A. Mnih, “Bayesian Probabilistic Matrix the University of Southern California, Los An- Factorization Using Markov Chain Monte Carlo,” ICML ’05: Proc. geles. Currently, he is with the Chinese Uni- 25th Int’l Conf. Machine Learning, 2008. versity of Hong Kong. His research interests[49] R. Salakhutdinov and A. Mnih, “Probabilistic Matrix Factoriza- include machine learning, web intelligence, tion,” Advances in Neural Information Processing Systems, vol. 20, social computing, data mining, and multimedia pp. 1257-1264, 2008. information processing. In these research areas,[50] B. Sarwar, G. Karypis, J. Konstan, and J. Reidl, “Item-Based he has more 200 technical publications in journals and conferences. In Collaborative Filtering Recommendation Algorithms,” WWW ’01: addition, he has contributed more than 20 book chapters and edited Proc. 10th Int’l Conf. World Wide Web, pp. 285-295, 2001. volumes. Moreover, he has more than 30 research and applied grants.[51] D. Shen, M. Qin, W. Chen, Q. Yang, and Z. Chen, “Mining Web One notable patented system he has developed is the VeriGuide Query Hierarchies from Clickthrough Data,” AAAI ’07: Proc. 22nd System, which detects similar sentences and performs readability Nat’l Conf. Artificial Intelligence, pp. 341-346, 2007. analysis of text-based documents in both English and in Chinese to[52] L. Si and R. Jin, “Flexible Mixture Model for Collaborative promote academic integrity and honesty. He is an associate editor of the Filtering,” ICML ’03: Proc. 20th Int’l Conf. Machine Learning, 2003. IEEE Transactions on Neural Networks (TNN) and IEEE Computational[53] C. Silverstein, M.R. Henzinger, H. Marais, and M. Moricz, Intelligence Magazine (CIM). He is a member of the editorial boards of “Analysis of a Very Large Web Search Engine Query Log,” the Open Information Systems Journal, Journal of Nonlinear Analysis ACM SIGIR Forum, vol. 33, no. 1, pp. 6-12, 1999. and Applied Mathematics, and Neural Information ProcessingCLetters[54] N. Srebro and T. Jaakkola, “Weighted Low-Rank Approxima- and Reviews Journal (NIP-LR). He has also served as special issue tions,” ICML ’03: Proc. 20th Int’l Conf. Machine Learning, pp. 720- guest editor for Neurocomputing, International Journal of Intelligent 727, 2003. Computing and Cybernetics (IJICC), Journal of Intelligent Information[55] J.-T. Sun, D. Shen, H.-J. Zeng, Q. Yang, Y. Lu, and Z. Chen, “Web- Systems (JIIS), and International Journal of Computational Intelligent Page Summarization Using Clickthrough Data,” SIGIR ’05: Proc. Research (IJCIR). Currently, he is serving the Neural Network Technical 28th Ann. Int’l ACM SIGIR Conf. Research and Development in Committee (NNTC) and the Data Mining Technical Committee under the Information Retrieval, pp. 194-201, 2005. IEEE Computational Intelligence Society (formerly the IEEE Neural[56] M. Theobald, R. Schenkel, and G. Weikum, “Efficient and Self- Network Society). He is a senior member of the IEEE and a member of Tuning Incremental Query Expansion for Top-k Query Proces- the ACM, International Neural Network Society (INNS), and Asian sing,” SIGIR ’05: Proc. 28th Ann. Int’l ACM SIGIR Conf. Research and Pacific Neural Network Assembly (APNNA). He is also a member of the Development in Information Retrieval, pp. 242-249, 2005. Board of Governors of INNS and a vice-president and governing board ´[57] B. Velez, R. Weiss, M.A. Sheldon, and D.K. Gifford, “Fast and member of APNNA. Effective Query Refinement,” ACM SIGIR Forum, vol. 31(SI) pp. 6- 15, 1997. Michael Rung-Tsong Lyu received the BS[58] L. von Ahn and L. Dabbish, “Labeling Images with a Computer degree in electrical engineering from National Game,” CHI ’04: Proc. SIGCHI Conf. Human Factors in Computing Taiwan University, Taipei, China, in 1981, the Systems, pp. 319-326, 2004. MS degree in computer engineering from the http://ieeexploreprojects.blogspot.com[59] X. Wang and C. Zhai, “Learn from Web Search Logs to Organize University of California, Santa Barbara, in 1985, Search Results,” SIGIR ’07: Proc. 30th Ann. Int’l ACM SIGIR Conf. and the PhD degree in computer science from Research and Development in Information Retrieval, pp. 87-94, 2007. the University of California, Los Angeles, in[60] J.-R. Wen, J.-Y. Nie, and H. Zhang, “Query Clustering using User 1988. He was with the Jet Propulsion Labora- Logs,” ACM Trans. Information Systems, vol. 20, no. 1, pp. 59-81, tory as a technical staff member from 1988 to 2002. 1990. From 1990 to 1992, he was with the[61] J. Xu and W.B. Croft, “Query Expansion using Local and Global Department of Electrical and Computer Engineering, The University of Document Analysis,” SIGIR ’07: Proc. 19th Ann. Int’l ACM SIGIR Iowa, as an assistant professor. From 1992 to 1995, he was a Conf. Research and Development in Information Retrieval, pp. 4-11, member of the technical staff in the applied research area of Bell 1996. Communications Research (Bellcore), Morristown, New Jersey. From[62] H. Yang, I. King, and M.R. Lyu, “DiffusionRank: A Possible 1995 to 1997, he was a research member of the technical staff at Bell Penicillin for Web Spamming,” SIGIR ’07: Proc. 30th Ann. Int’l Laboratories, Murray Hill, New Jersey. In 1998, he joined The Chinese ACM SIGIR Conf. Research and Development in Information Retrieval, University of Hong Kong, where he is now a professor in the pp. 431-438, 2007. Department of Computer Science and Engineering. He is also a[63] Y.-H. Yang, P.-T. Wu, C.-W. Lee, K.-H. Lin, W.H. Hsu, and H. founder and director of the Video over InternEt and Wireless (VIEW) Chen, “ContextSeer: Context Search and Recommendation at Technologies Laboratory. He has published 330 refereed journal and Query Time for Shared Consumer Photos,” Proc. 16th ACM Int’l conference papers in these areas. He was the editor of two book Conf. Multimedia, pp. 199-208, 2008. volumes: Software Fault Tolerance (New York: Wiley, 1995) and The[64] B. Zhang, H. Li, Y. Liu, L. Ji, W. Xi, W. Fan, Z. Chen, and W.-Y. Handbook of Software Reliability Engineering (Piscataway, N.J.: IEEE Ma, “Improving Web Search Results Using Affinity Graph,” SIGIR and New York: McGraw-Hill, 1996). He initiated the First International ’05: Proc. 30th Ann. Int’l ACM SIGIR Conf. Research and Development Symposium on Software Reliability Engineering (ISSRE) in 1990. He in Information Retrieval, pp. 504-511, 2005. was the program chair for ISSRE 1996 and general chair for ISSRE 2001. He was also PRDC 1999 program cochair, WWW10 program Hao Ma received the BEng and MEng degrees cochair, SRDS 2005 program cochair, PRDC 2005 general cochair, in the School of Information Science and ICEBE 2007 program cochair, and SCC 2010 program cochair. He will Engineering from Central South University in be the general chair for DSN 2011 in Hong Kong. He was on the 2002 and 2005, respectively, and the PhD editorial boards of the IEEE Transactions on Knowledge and Data degree from the Computer Science and Engi- Engineering, IEEE Transactions on Reliability, Journal of Information neering Department, The Chinese University of Science and Engineering, and Wiley Software Testing, Verification and Hong Kong (CUHK) in 2010. He worked as a Reliability Journal. His research interests include software reliability system engineer at Intel Shanghai before he engineering, distributed systems, fault-tolerant computing, mobile and joined CUHK as a PhD student in November sensor networks, Web technologies, multimedia information proces- 2006. His research interests include information sing and retrieval, and machine learning. He is an IEEE fellow, anretrieval, data mining, machine learning, social network analysis, AAAS fellow, and a croucher senior research fellow. He was alsorecommender systems, human computation, and social media analysis. named by the IEEE Reliability Society as the 2010 Engineer of the Year.